The concept of an “electronic nose” has been an active area of research for some decades. Researchers have been trying to provide such a device e.g., by attempting to couple an array of chemical odor sensors with a pattern-recognition system. NASA, for instance, has been trying to develop such a nose to detect the presence of gases such as ammonia that may be poisonous to astronauts. Ammonia is integral to life on the space station because it carries heat originating inside the station through pipes outside to space. However, ammonia is poisonous and a leak must be detected quickly and stopped. Humans are only capable of detecting ammonia at about 50 ppm, although it can be dangerous at a concentration of only a few parts per million. A suitable sensor, or electronic nose capable of detecting ammonia at such low concentrations is needed.
Considerable interest has been generated by the Department of Homeland Security about the use of electronic devices in detecting volatile compounds to prevent explosive, chemical, or biological attacks. Screening methods developed to detect explosive or toxic chemicals that may be carried through an airport or seaport are, in many ways, a first line of defense in protecting against such attacks. Such methods currently include laboratory analyses of suspected drugs, drug-sniffing dogs, and the ubiquitous X-ray machine. For most laboratory methods to be performed, a great deal of time must be spent in preparing samples for analysis. Small, portable devices would likely reduce the time needed to detect potential threats
Previous attempts at making electronic noses generally follow the same principle, coupling an array of chemical detectors with pattern recognition systems. However, they differ with respect to the selection of sensors. Common sensor designs include mass transducing, such as quartz microbalance, surface acoustic wave transducers, chemoresistors, and hybrids of such. In any event, it is greatly desired that sensors used for electronic noses and molecular detection exist and function on a very compact—even molecular—scale and exhibit very good electronic properties.
Since their discovery, carbon nanotubes have stimulated widespread scientific research due to their promising electronic and mechanical properties. Their electronic properties, for instance those exhibited in semiconducting nanotubes, suggest conductivity characteristics that change as a result of local electrical field strength. Martel, et al., “Single and Multi-Wall Nanotube Field Effect Transistors,” Applied Physics Letters, Volume 73, Number 17, 26 Oct., 1998, have constructed molecular structures that exhibit field-effect transistor (FET) characteristics using nanotubes that exhibit variable electrical conductance attributes. An FET, which is known to the art per se, may be described as a current device in which current flows in a channel between two electrodes where the effective resistance of the channel may be controlled using a gate electrode which alters the strength of the electric field in the channel.
The use of single walled carbon nanotubes (SWNT) as chemical sensors has been explored in the works of Chen R J, et. al., (2004) J. Am. Chem. Soc. 126:1563-8; and, Bradley K, et. al. (2003) Phys. Rev. Lett., 91:218301. The one-dimensional carbon cage structure of semiconducting SWNTs makes their physical properties exquisitely sensitive to variations in the surrounding electrostatic environment, whether the SWNTs are suspended in liquid or incorporated in to the FET circuits on a substrate. (Kong J et al. (2000) Science 287″622-5; Freitage M et al. (2002) Phys Rev Lett 89 art. No. 216801; and, Pengfei Q F et al. (2003) Nano Lett. 3:347-51). Bare and polymer-coated SWNTs have been reported to be sensitive to various gases, but SWNTs functionalized with biomolecular complexes are believed to hold the greatest promise as molecular probes and sensors for chemicals that do not interact, or interact only weakly with bare carbon nanotubes (Chopra S et al. Appl. Phys. Lett. (2003) 83:2280-2; Li J et al. (2003) Nano Lett. 3:929-33; Novak J P et al (2003) Appl. Phys. Lett. 83:4026-8; Valentini L et al. (2003) Appl. Phys. Lett. 82:961-3; Bradley K et al. (2003) Appl. Phys. Lett. 83:3821-3; Snow E S et al. (2005) Science 307:1942-5; Wong S S et al. (1998) Nature 394:52-5; Williams K A et al. (2002) Nature 420:761; Chen R J et al. (2003) Proc. Natl. Acad. Sci. USA 100:4984-9; and, Barone P W et al. (2005) Nat. Mater. 4:86-92.)
Derivatized SWNT-FETs are attractive as electronic readout molecular sensors due to their high sensitivity, fast response time, and compatibility with dense array fabrication (Pegfei Q F et al. 2003). Derivatized semiconductor nanowires have similar performance advantages, and recent work indicates that they hold promise as gas and liquid-phase sensors (Zhang D et al. (2004) Nano Lett. 4:1919-24; Hahm J I et al. (2004) 4:51-4; and, Wang W et al. (2005) Proc. Natl. Acad. Sci. USA 102:3208-12.)
It is desirable to functionalize SWNT sensors in a way to achieve robust, reproducible decoration, with concomitant molecular flexibility to provide sensitivity to a wide spectrum of analytes. To avoid degrading the high quality electronic properties of the SWNT-FET, it is also desirable that the functionalization be non-covalent.
Nucleic acids are intriguing candidates for use as in the molecular targeting layer of SWNT-FETs. Nucleic acids are advantageous in that they can be engineered for affinity to a wide spectrum of targets, including small molecules, proteins, and other nucleic acids (Patel D J et al. (1997) J. Mol. Biol. 272:645-64; and, Breaker R R (2004) Nature 432:838-45). Single-stranded DNA (ssDNA) labeled with fluorescent tags has shown promise as a sensor of volatile compounds. In U.S. Patent Application Publication No. 2004/0101851 A1, White and Kauer discuss that fluorescence from tagged ssDNA changes when a bulk layer of a molecule is exposed to odor samples. In the White and Kauer system, dyes interclated within the DNA change fluorescence upon exposure to the odor compound relative to the intensity observed upon exposure to clean air controls. ssDNA possesses the additional advantage of having a high affinity for SWNTs due to a favorable pi-pi stacking interaction, and thus can facilitate functionalization of the carbon nanotube (Zheng M et al. (2003) Nature Mater. 2:338-42). Moreover, because different ssDNA strands show different response characteristics, the unique chemical structures of specific ssDNA oligomers grant the sensors made from the tagged DNA customizable binding properties.
A significant advance in the art would be to provide chemical sensors that are compact and capable of detecting volatile compounds with all-electronic readout. It is desirable that such sensors be capable of easy modification, in order to provide a wider array of potential sensitivities as well as to be able to enhance the sensitivity to known analytes. Similarly, as the sensors currently available suffer from the major drawback of irreversible adsorption, it is desirable to provide sensors that are capable of self-regeneration in order to prolong the usefulness of the device and save on cost. Also needed are methods of detecting such volatile compounds using a device comprising individual sensors or arrays made of compact chemical sensors.